release of off screen buffers

[maybites]

the off screen buffer needs to be more carefully handled. The current implementation is probably creating a memory leak.

https://docs.blender.org/api/blender2.8/gpu.types.html#gpu.types.GPUOffScreen https://docs.blender.org/api/blender2.8/gpu.html#copy-offscreen-rendering-result-back-to-ram